Constant temperature anemometers (CTAs) are known to the prior art. In such devices, a heated resistance element serves as a sensing element. The sensing element has a temperature coefficient of resistance and is maintained at constant resistance and, thus, temperature.
Typical prior art constant temperature anemometers place the sensor in one leg of a bridge circuit with a feedback circuit, including an amplifier, being employed to maintain balance on the bridge. An example of such anemometer is illustrated in FIG. 1 wherein the sensing element or sensor is designated at 10 and forms one leg of a bridge, the other bridge legs being formed by resistances 11-13. An operational amplifier 14 has it inputs connected to two separate junctions on the bridge circuit with its output connected to a third bridge junction. The fourth bridge junction is grounded. Resistances 11 and 12 determine the "bridge ratio" while the resistance 13 determines the operating resistance of the sensor.
If the environmental conditions surrounding the sensor 10 of FIG. 1 decrease its heat loss in the wind tunnel, the sensor tends to increase in temperature. The operational amplifier 14 senses this as a bridge off-balance and decreases its output until the bridge again approaches balance. For low frequencies, operational amplifier 14 having high gain, maintains the bridge very close to balance over a wide range of current flow through the sensor 10.
A major problem with constant temperature anemometers of the type illustrated in FIG. 1 is the maintenance of stable operation at high frequencies. Ideally, this should be accomplished with simple controls and without restricting the type of sensor that may be employed. Any adjustments should also minimize the change in maximum frequency response as the operating point changes.
Although the circuit of FIG. 1 appears quite simple, an analysis of its stability as a constant temperature anemometer is quite complex. One analysis is provided by P. Freymuth, in an article entitled "Frequency Response and Electronic Testing for Constant Temperature Hot-Wire Anemometers," Journal of Physics E: Scientific Instruments, Vol. 10, 1977. In essence, two controls are necessary for optimization of this CTA system, one to "trim" the reactance of the bridge, and the other to influence the closed loop gain of the system at high frequencies.
A prior art approach to the optimization discussed above is illustrated in FIG. 2. Throughout FIGS. 1-3, like reference numerals indicate the corresponding circuit elements. In FIG. 2, a variable inductor 15 is provided to adjust the reactance of the bridge while the gain of the operational amplifier 14 is "shaped" by an RC circuit designated generally at 16 in the feedback loop. The RC circuit 16 is formed of a capacitance 17 and a variable resistance 18. At high frequencies, the gain of operational amplifier 14 is reduced to a value established by the resistance of the variable resistor 18. While it provides an improvement over the anemometer shown in FIG. 1, the circuit of FIG. 2 is not stable when the cut-off frequency of the sensor approximates that of the RC circuit 16. In addition, the adjustments to the inductance 15 and the RC circuit 16 are coupled--a change in one changes the optimum setting for the other--which increases the time required for optimization.